Transverse rarefaction of a layer behind the front of a longitudinal wave of nonlinear thermal conductivity and features of plasma formation inside the target cavity

A solution of the two-dimensional problem is presented for a transverse rarefaction wave of a plane layer behind a wavefront of nonlinear thermal conductivity produced by an instantaneous cylindrical energy source with its axis normal to the layer surface. The matter can rarefy through the free surfaces of the layer or through holes in the boundary walls, which are coaxial with the energy source axis. Analytic solutions are obtained describing the formation of the rear boundary in the heated zone due to transverse matter rarefaction. The energy fraction transferred to the energy of hydrodynamic motion is also determined. Models of plasma formation inside the inner target cavity under the action of pulsed energy sources are considered as applications of the approach suggested. Requirements on the source and target parameters are formulated for efficient matter heating with minimum energy losses caused by hydrodynamic rarefaction of the matter. Translated from Preprint No. 14 of the P. N. Lebedev Physical Institute, Moscow (1998).     

CH薄膜非平衡辐射烧蚀的特性

 用一维多群辐射输运流体力学RDMG程序数值模拟研究了在神光-II黑腔辐射源条件下，非平衡辐射烧蚀CH薄膜的过程，给出了与平衡辐射烧蚀不同的烧蚀图像，得到了非平衡辐射烧蚀相关的数值定律。     

CH薄膜非平衡辐射烧蚀的特性

用一维多群辐射输运流体力学RDMG程序数值模拟研究了在神光-II黑腔辐射源条件下,非平衡辐射烧蚀CH薄膜的过程,给出了与平衡辐射烧蚀不同的烧蚀图像,得到了非平衡辐射烧蚀相关的数值定律。     

Optodynamic energy conversion efficiency during laser ablation on metal surfaces measured by shadow photography

In this investigation, we used high-speed shadow photography to observe fast optodynamic phenomena such as shock waves and the ablation of flat metal surfaces. These phenomena were induced in air by a Q-switched Nd:YAG laser (λ = 1,064 nm) with a pulse duration of 4 ns and an excitation pulse energy between 10 and 55 mJ. For a good spatial resolution of the shadowgraphs, we used short illumination pulses (30 ps) from a frequency-doubled Nd:YAG laser (λ = 532 nm). Using the shadowgraphs of the shock wave expansion into a half-space, we measured the optodynamic energy conversion efficiency, defined as the ratio between the mechanical energy of the shock wave and the excitation pulse energy. This efficiency increases with an increasing excitation pulse energy. We also present the characteristic shadowgraphs of the ablation of a black-painted metal surface, where the macroscopic material particles are clearly visible. They follow the shock wave and eventually overtake it. As a result, the shape of the shock wave, which normally expands concentrically into the half-space, has an altered form. The presented results reveal the phenomenon of the laser ablation of coated metal surfaces.     

Numerical Simulation of Shock Wave Generation for Ignition of Precompressed Laser Fusion Target

In this work, we investigate the formation of a converging shock wave in a homogeneous spherical target, whose outer layer was heated by a flux of monoenergetic fast electrons of a given particle energy. Ablation pressure generating the wave forms at spherical expansion of a layer of a heated substance, whose areal density remains constant throughout the entire heating process and equal to the product of the initial heating depth and density of the target. The studies are carried out based on numerical calculations using a one-dimensional hydrodynamic code as applied to ignition of a precompressed target by a shock wave (shock ignition), one of the most promising techniques of laser fusion ignition.     

Ablation of metals by ultrashort laser pulses: Theoretical modeling and computer simulations

Laser ablation of metals by femto- and picosecond pulses is analytically and numerically studied within the framework of the plasma model for the ablated material. Ablation is initiated by high-power thermal and hydrodynamic waves which propagate into the irradiated material. Analytical expressions for the thermal ablation and for the ablation by the shock wave are obtained. Numerical simulations with the computer code RAPID are in good agreement with analytical results. Translated from Preprint No. 64 of the P. N. Lebedev Physical Institute, Moscow (1996).     

微激光冲击DZ17G合金的表面完整性影响研究

为了在不影响柱状晶组织的前提下改善DZ17G定向凝固合金的力学性能,采用微激光冲击强化方法进行表面处理,通过X射线衍射、扫描电子显微镜、透射电子显微镜和显微硬度计,测试分析微激光冲击对DZ17G定向凝固合金表面完整性的影响。试验结果表明：在水下无吸收保护层微激光冲击处理后,合金表面发生了烧蚀、熔融,1次冲击后形成光滑熔融区,但随着冲击次数增加而形成了大量微小烧蚀孔洞和难熔颗粒;表层组织仍由和两相组成,柱状晶内形成了高密度位错和位错缠结,但未发生晶粒细化;硬度在深度上呈梯度分布,冲击1次后硬化层深度仅为100 m,表面硬度值达到503 HV,提高了22.7%,而且硬度值和硬化层深度都随着冲击次数增加而增大。     

New ablation experiment aimed at metal expulsion at the hydrodynamic regime

The effects of nanosecond visible laser on metallic materials have been studied experimentally. High laser energies (>10¹³ W/cm²) created a hydrodynamic regime, where the ablation pressure and the ensuing shock wave are the main mechanisms for material expulsion. Plasma shielding caused a constant material removal despite the increase of energy, while the increase of number of pulses resulted in an almost linear increase of the crater volume, despite the lower depths reached with every subsequent pulse. Our results show that there is a correlation between ablation efficiency and material properties, namely ablation efficiency decreases with melting temperature and bulk modulus.     

微激光冲击DZ17G合金的表面完整性影响研究

为了在不影响柱状晶组织的前提下改善DZ17G定向凝固合金的力学性能,采用微激光冲击强化方法进行表面处理,通过X射线衍射、扫描电子显微镜、透射电子显微镜和显微硬度计,测试分析微激光冲击对DZ17G定向凝固合金表面完整性的影响。试验结果表明:在水下无吸收保护层微激光冲击处理后,合金表面发生了烧蚀、熔融,1次冲击后形成光滑熔融区,但随着冲击次数增加而形成了大量微小烧蚀孔洞和难熔颗粒;表层组织仍由和两相组成,柱状晶内形成了高密度位错和位错缠结,但未发生晶粒细化;硬度在深度上呈梯度分布,冲击1次后硬化层深度仅为100 m,表面硬度值达到503 HV,提高了22.7%,而且硬度值和硬化层深度都随着冲击次数增加而增大。     

Instanton-like self-dual solution to lattice Euclidean Gravity: Difference from Eguchi–Hanson solution to continuous gravity

An analytical solution has been obtained for the spherical isothermal expansion of the outer layer of a ball whose mass increases at a constant areal density of the heated layer, which is equal to the product of the initial values of the depth of heating and the density of the layer for the entire time of expansion into vacuum. This solution differs from the known solution for the isothermal spherical expansion of a given mass of a material in a slower decrease in the density and, as a result, in the pressure of the expanding material with the time. In particular, it describes the expansion of the boundary layer of the ball heated by a flow of fast electrons in application to the problem of the ignition of an inertial confinement fusion target by a shock wave induced because of the heating of the target by the flow of laser-accelerated fast electrons (shock ignition).     

脉冲宽度对准单能质子能量的影响

使用1维粒子模拟程序LPIC++模拟了不同脉冲宽度的P极化超短强激光脉冲与稠密冷等离子体靶的相互作用。模拟结果表明：激光有质动力首先推动表层等离子体形成无碰撞激波,然后激波向靶内传播,俘获靶内的质子并将其反射加速,反射质子的速度约为离子声速的2倍。当使脉冲宽度与靶厚互相匹配时,也就是靶厚等于离子声速与激光脉冲宽度的乘积时,可以得到能散为30%左右的单能束。与激光脉冲宽度与靶厚不匹配相比,准单能质子束的能量得到有效提高,提高幅度达60%。同时,为弄清2维效应的影响,还使用2维粒子模拟程序OOPIC进行了模拟,与1维所获得结果基本一致。     

Performance analysis of a new biolistic gun using high power laser irradiation

Impingement of a high power laser pulse (above 10⁹ W/cm²) on a metal foil causes ablation, which is characterized by a rapid expulsion of matter and initiation of a strong shock wave inside the solid metal. The shock propagates through the foil and reverberates on the rear side causing instant deformation of the foil, whose surface is treated with micro particles prior to ablation. Based on this principle of micro particle ejection, we develop a new biolistic gun with improved controllability, stability, efficiency of our previous system, and perform characterization of the penetration shapes at varying confinements and energy levels. The confinement media include BK7 glass, water, and succulent jelly (ultrasound gel). Biological tissue was replicated by a gelatin-water solution at a 3% weight ratio. Present data show that confinement effect results in a conspicuous enhancement of penetration reached by 5 μm cobalt micro particles. Also, there exists an optimal thickness at each energy level when using liquid confinement for enhanced particle delivery.     

Optodynamic study of multiple pulses micro drilling

This paper describes an analysis of pulsed lasers micro-drilling of different metals. Study focuses to an optodynamic phenomenon which appears as thermal effects induced by laser light pulses and leads to dynamic process manifested as ultrasonic shock waves propagating into the sample material. The shock waves are detected by a non-contact optical method by using arm compensated Michelson. Monitoring of the main parameters of the micro drilling such as material ablation rate and efficiency was realized by analysis of the optodynamic signals. The process is characterized by decreasing ablation rate that leads to the finite hole depth. The experimental part of study comprehends a comparison between various metals. In order to describe decreasing ablation rate a theoretical model based on the energy balance is proposed. It considers the energy/heat transfer from the laser beam to the material and predicts a decreasing drilling rate with an increasing number of successive laser pulses. According to the proposed model, the finite depth of the hole appears as a consequence of the increasing surface area through which the energy of the laser beam is conducted away to the material around the processed area. Decreasing ablation rate and the finite hole depth predicted by model were in good agreement with the experimental results.     

Simulation of ultrashort double-pulse laser ablation

In this paper, we study the mechanisms of femtosecond double-pulse laser ablation of metals. It was previously shown experimentally that the crater depth monotonically drops when the delay between two successive pulses increases. For delays longer than the time of electron-ion relaxation the crater depth can be even smaller than that produced by a single pulse. The results of the performed hydrodynamic simulation show that the ablation can be suppressed due to the formation of the second shock wave. The modeling results of the double-pulse ablation obtained for different delays correlate with the experimental findings.     

量纲分析法求解辐射热传导自相似解

通过量纲分析,将辐射流体方程简化为一阶常微分方程组的形式,进而结合数值计算求得热波烧蚀自相似解。流体方程涉及的内能、状态方程表达式采用依赖于温度、密度两个参量的形式,且参数可调,适用性较广。源边界条件可为恒温、恒流、恒压、常密等情况。以恒温源为例,给出了X射线加热Au壁时烧蚀质量与烧蚀压的定标关系,与文献结果及MULTI辐射流体程序计算结果吻合较好。     

Laser ablation of aqueous solutions with spatially homogeneous and heterogeneous absorption

The ablation efficiency of aqueous solutions with different concentrations and spatially homogeneous (CuCl₂ solution) and heterogeneous (ink solution) absorption was studied as a function of the pulse-energy fluence (Nd:YAG laser, λ=1064 nm, τ_p = 20 ns). The latter was varied over a wide range from 0.15 J/cm² to 8.00 J/cm². The ablation threshold of solutions with heterogeneous absorption was found to be much lower (3 to 4 times) than the ablation threshold of solutions with homogeneous absorption and with the same average absorption coefficient. The ablation efficiency of heterogeneous solutions was higher by more than an order of magnitude. It was found that the ablation efficiency increases drastically for both types of solutions as the pulse energy fluence was raised to exceed the ablation threshold by 2 or 3 times. At such energy fluences, along with small droplets, larger droplets (1.5–2 mm cross section) could be ejected. This points to the ablation of solutions being affected by a hydrodynamic shock formed as a result of the pulsed recoil pressure excerted by the ablation products. The differences between the ablation processes for solutions with homogeneous and heterogeneous absorption as well as the hydrodynamic destruction at high energy fluences are discussed.     

Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (<1 PW/cm²) femtosecond laser pulses have been detected by observing the propagation of a shock wave in air from the ablated surface to a broadband piezoelectric receiver. The estimated initial pressure and velocity of the shock wave (ablation plume) agree well with data obtained earlier by various methods for shock waves propagating inside ablated targets.     

大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.     

The numerical study of shock-induced hydrodynamic instability and mixing

Based on multi-fluid volume fraction and piecewise parabolic method (PPM), a multi-viscosity-fluid hydrodynamic code MVPPM (Multi-Viscosity-Fluid Piecewise Parabolic Method) is developed and applied to the problems of shock-induced hydrodynamic interfacial instability and mixing. Simulations of gas/liquid interface instability show that the influences of initial perturbations on the fluid mixing zone (FMZ) growth are significant, especially at the late stages, while grids have only a slight effect on the FMZ width, when the interface is impulsively accelerated by a shock wave passing through it. A numerical study of the hydrodynamic interfacial instability and mixing of gaseous flows impacted by re-shocks is presented. It reveals that the numerical results are in good agreement with the experimental results and the mixing growth rate strongly depends on initial conditions. Ultimately, the jelly layer experiment relevant to the instability impacted by exploding is simulated. The shape of jelly interface, position of front face of jelly layer, crest and trough of perturbation versus time are given; their simulated results are in good agreement with experimental results.     

Two-dimensional analysis of the effect of an electrode layer on surface acoustic waves in a finite anisotropic plate

The effect of a metal layer over an elastic substrate on surface acoustic wave propagating in the structure can be evaluated precisely for semi-infinite solids and infinite plates, but there is no accurate analytical solution if the finite size of the plate has to be considered. By expanding displacements with eigensolutions of surface acoustic waves in a semi-inifite solid, a set of two-dimensional equations similar to the Mindlin plate theory are obtained. Then for a thin electrode layer, the effect is considered through the approximation of displacements in the metal layer with the ones in the substrate, and an integration over the thickness incorporated the properties of the metal layer into equations through the modification of material properties with the decaying indices of surface acoustic waves and the thickness of the metal layer. Using AT-cut quartz crystal as the substrate, we present the effect of silver electrode layers of finite thickness on the phase velocity of propagating surface acoustic waves.